This invention relates to a method and apparatus for the non-contact measurement of a dimension of a piece of material. More particularly, the invention provides an apparatus and method for measuring the length or width of a piece of aluminum sheet or plate.
Numerous systems are known for measuring the length of a piece of material, such as hot aluminum sheet or plate. All of the known systems have disadvantages when compared with the system provided by the present invention.
One type of system requires strict control of lighting. Such system is difficult to use with aluminum because of the spectral characteristics of aluminum sheet and plate and the possibility of false readings because of surface defects and lubricant residues. There are high initial installation costs and continuous maintenance requirements, especially with installations designed to measure distances between a few feet and 150 feet.
Another type of measurement system utilizes a contact member designed to roll on the material being measured. A problem with this type of system is slippage because of lubricants on the materials being measured. In addition, it is difficult to obtain materials to use with hot aluminum plate (approximately 800.degree. F.).
Another type of measurement system requires close proximity installation to the material being measured. The problem with a system of this type is the frequency of accidents associated with the measurement of aluminum plate. Plates occasionally fall off of a conveyor, for instance when the plate momentarily adheres to a roller feeding the plate to the measuring installation. Varying shapes of plate ends, such as "alligatoring", also make such close installations impractical.
Low resolution measurement systems also are difficult to use with aluminum because of the need for multiple sensors to read length, which results in high initial installation costs to ensure adequate resolution along a 150-foot long system. The upkeep and maintenance of such a system would also be extremely expensive.
Systems also are known relying on the measurement of speed. These systems read product speed and integrate to obtain the length. Such systems are inappropriate for long lengths and require extremely close monitoring of speed. This system also is difficult and time consuming to use because of the need to run the plate back and forth under a speed sensor to obtain desired measurements.
Still another type of measurement system measures a relatively short range and adds it to a known length. Because of the need for measuring consistently plate lengths that may vary between 6 feet and 160 feet, such system is impractical in the present situation. Such system would require a different setup for each measurement because of the relatively small measurement range. In addition, a system of this type must be relatively near the conveyor which would subject it to possible damage by plate falling off the conveyor.
Still another type of measurement system utilizes a moving camera to obtain measurements. This system requires a substantially continuous track that is installed either adjacent to the conveyor or on a wall. The system would be extremely unwieldy electrically and mechanically for a 160 foot measurement range and would require long measurement times because of the distances to be traveled. In addition, problems could be encountered with ensuring consistent calibration of such a system.
A final type of system equalizes a combination of temperatures and camera saturation. When measuring steel, a plurality of heat sensors are positioned along the measurement range and are combined with camera saturation. Similar temperature measurement of aluminum is difficult, relatively inaccurate, and hard to keep in calibration.